JSCE Journal of Disaster FactSheets, FS2022-E-0001, 2022

Widespread ground deformation over the Palu Basincaused by the 2018 Sulawesi, Indonesia Earthquake
Kazuo KONAGAI1, Ryoichi FURUTA2, Takashi KIYOTA3
1

Fellow of JSCE, Professor Emeritus, University of Tokyo(1-21-4-517 Wakaba, Shinjuku-ku, Tokyo 160-0011, Japan)
E-mail: konagai@iis.u-tokyo.ac.jp
2
Member of JSCE, Group Leader, Social Infrastructure Group, Remote Sensing Technology Center of Japan(3 -17-1 Toranomon, Minato-ku, Tokyo 105-0001, Japan)
E-mail: furuta_ryoichi@restec.or.jp
3
Member of JSCE, Associate Professor, Institute of Industrial Science, University of Tokyo
(4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan)
E-mail: kiyota@iis.u-tokyo.ac.jp

Key Facts
・ Hazard Type: Earthquake
・ Date of disaster: Sept. 28th, 2018
・ Location of Survey: Palu, Central Sulawesi, Indonesia
・ Date of the field survey: Not Applicable・ Survey tools: Digital Terrain Models, InSAR・ Key findings:
1) Pre and Post-earthquake Digital Terrain Models (DTMs) covering Palu Basin suggested that theentire stretch of the basin, mainly west of the irrigation canal, seemed to have deformed, not tomention the large flow slide sites.
2) Line-of-sight (LOS) deformations over the Palu Basin obtained from ALOS 2/PALSAR 2 at
two different days after the earthquake (October 12, 2018, and January 4, 2019) showed that theflat land has slowly and steadily been deforming over the three months.
Key Words : 2018 Sulawesi Earthquake, long-lasting ground displacement buildup, lateral slides

distance at each site was reportedly several hundred
meters or more. As of October 9, 2018, at least 671
were on the list of missing, but the National DisasterManagement Agency said the mudflows may haveswallowed up many more people when the flows
engulfed houses in these villages (Aprilia, K.3)).
Though the administration's final count of victims
has finally reached 4,340 (United Nations Office for
the Coordination of Humanitarian Affairs, OCHA4)),
the actual death toll due to the combined effects ofthe earthquake and tsunami might never be known(Aprilia, K.3)).
There are various theories for the lateral-spreadinitiations and motions discussed by many
researchers (Kiyota et al.2), Rohit, D.1)). They have acommon opinion that liquefiable deposits andmultiple capping layers might have triggered the flowslides. Bradley, K.5) reported that aqueduct-supported
cultivation, primarily of wet rice, raised the water

1. INTRODUCTION
A devastating Mw 7.5 earthquake struck Central
Sulawesi, Indonesia, on September 28, 2018,
resulting in over 4,000 fatalities and severe damage
to several areas in and around Palu City. Sulawesi isa vast island of Indonesia with four spindly arms
spinning outward. Palu city is at the inner-most partof Palu Bay that cuts in the northernmost arm joint.
The south-north trending Palu Basin that extends
straight to Palu Bay features the unique terrain of theepicentral area. The epicentral area is a typical pullapart tectonic basin with a thick alluvial deposit
filling the valley. This earthquake's most spectacularand devastating aspect was lateral slides of the almost
flat alluvial soil deposit along the west and eastbounds of the basin. Lateral spreads were the mostserious in Balaroa, Petobo, Jono Oge, and Sibalaya(Rohit, D.1), Kiyota et al.2)). The maximum flow1

JSCE Journal of Disaster FactSheets, FS2022-E-0001, 2022

Post-quake DTM – Pre-Quake DTM(incl. © Maxar Technologies, NTT DATA Corporation)

Sulawesi

PetoboJono Oge

Balaroa

PetoboJono Oge
Analyzed area. Contour lines were extracted after 4th iterationof 2D-KZ filter.

Googleterrain

Fig. 1 Difference in the digital terrain models (incl. © Maxar Technologies, NTT DATA Corporation) before andafter the earthquake: The Two-Dimensional (2D) Kolmogorov–Zurbenko (KZ) filter was applied to the image tosmear the rapid intensity changes and draw smoother contour lines. The contour lines are drawn with the unit andthe interval set at the “meter” and “0.2 m,” respectively. Yellow polygons show locations of extensive flow slides.

deformations over the Palu Basin obtained fromALOS 2/PALSAR 2 on two different days after theearthquake (October 12, 2018, and January 4, 2019).
This article describes the preliminary results of ouranalyses of the two topographic data sets.

table near ground level, saturating sandy alluvial
soils. Thus, the devastating lateral spreading in thePalu Valley could have been a direct consequence ofirrigation. However, the cause of flow-slide cannot
be determined solely by the presence of the irrigation
channel because there was no primary irrigation
channel in Balaroa, the west side of Palu Valley(Kiyota et al.2)).
For a thorough discussion of the impact of soilspreading, it is essential to look at the big picture of
the soil spreading over the whole Palu Basin. We alsoneed to keep a watchful eye on if the ground has been
deforming since the devastating events. Though thepost-quake movements of the land can be small, thedeformation buildups may have affected lifeline
restoration operations in these areas. For thispurpose, we use two sets of topographic data; (1)Digital Terrain Models of the Palu Basin before andafter the earthquake and (2) Line-of-sight (LOS)

2. IMMEDIATE GROUND
DEFORMATIONThe Digital Terrain Models (AW3D DTMs, 2 m
resolution, NTT Data, RESTEC6)) before and after
the earthquake are random gatherings of various
source data sets at different times. The DTM before
the earthquake covers the period from January 1,2010, to September 27, 2018, while the post-quake
DTM covers the period from September 28, 2018, toNovember 20, 2018. Differences in spatial coverageand levels of detail often create discontinuities within
the overlapping areas of the source DTMs and along2

JSCE Journal of Disaster FactSheets, FS2022-E-0001, 2022

PetoboJono Oge

Off-Nadir angle
28.6°

LOS displacements. Contour lines were extracted after 4th iteration of2D-KZ filter (DInSAR processed by ©RESTEC, Included ©JAXA)
Post-quake DTM – Pre-Quake DTM(incl. © Maxar Technologies, NTT DATA Corporation)

Fig. 2 Line-of-sight (LOS) deformations over the same area as Fig. 1 from ALOS 2/PALSAR 2 on two differentdays after the earthquake (October 12, 2018, and January 4, 2019): DInSAR processed by ©RESTEC, Included©JAXA. The Two-Dimensional (2D) Kolmogorov–Zurbenko (KZ) filter was applied to the image to smear therapid intensity changes and draw smoother contour lines. The contour lines are drawn with unit and interval set atthe “millimeter” and “5 mm,” respectively. Yellow polygons show locations of extensive flow slides.

their boundaries. Therefore, calibration wassupposed to have been made with precise reference
points given on the ground. However, amid theCOVID-19 pandemic, which did not allow any fieldworks, we subtracted the pre-quake DTM from thepost-quake DTM to evaluate the vertical componentsof the quake-induced ground displacement. To avoid
rapid scatters in the image intensity, the difference of
the DTMs was first resampled for a burred image of
10 m resolution. Then, the Two-Dimensional (2D)
Kolmogorov–Zurbenko (KZ) filter (Zurbenko7),Yang, W.8)) was applied to the image to smear therapid intensity changes further. The 2D-KZ filter is aseries of iterations of a moving average square filter(2L+1)×(2L+1), where L is a positive integer. It has
thus two parameters, the half-length L of the squarewindow and the number of iterations k of the moving

average operation. Since the resampled DTM has a
10 m resolution and L was set at 5, the moving
average window is 110-m square.
Fig. 1 shows the 2D-KZ filtered image of theelevation change over the 8,600 m × 8,600 m flatland area south of Palu City. Note that the 4th
iteration of the moving average with the half-lengthof the window set at 50 m conservatively truncatesthe original target area of 9,000 m × 9,000 m by 400
m and 400 m in north-south and east-west directions.
The area includes two extensive flow slides at Petobo
and Jono Oge. The ground deformation is notexclusive to these large flow slide sites. Surprisingly,almost the entire analyzed area, particularly west of
the irrigation canal, shows a checkerboard elevation
change pattern. The pattern shows alternating
positive and negative values of elevation change3

JSCE Journal of Disaster FactSheets, FS2022-E-0001, 20222019). The obtained images suggested that the entire
stretch of the basin, mainly west of the irrigation
canal, seemed to have deformed, not to mention thelarge flow slide sites. If the widespread grounddeformation is attributed to soil liquefaction, theliquefied layer can remain soft, causing the ground to
deform gradually for an extended period. Theobserved
patternofLine-of-sight(LOS)
deformations over the Palu Basin shows that the flat
land has slowly been deforming over the threemonths (from October 12, 2018, to January 4, 2019),
backing up the hypothesis. These findings teach us
essential lessons to enter the immediate andfollowing stages of rehabilitations of liquefactiondevastated areas. We need to take necessary
measures considering the liquefied ground canremain soft for an extended period. Further studies
will be necessary for more quantitative discussions.

ranging over about -1.0 m or below to + 1.0 m or
more prominent. Though the calibrations of theDTMs are needed, these peak values are substantiallylarge enough to acknowledge the statisticalsignificance of the 2D-KZ filtered image of theelevation change. The positive and negative peaks
appear periodically at an average interval of about 1km. This pattern can reflect the subsurface soil profilethat can include liquefied layers.

3. POST-QUAKE CREEPING GROUND
DEFORMATION
If the widespread subsurface layer liquefies, thelayer can remain soft for an extended period causing
the ground surface to deform gradually (Kiyota etal.9)). For confirmation purposes, the authors haveobtained Line-of-sight (LOS) deformations over thesame area from ALOS 2/PALSAR 2 on two differentdays after the earthquake (October 12, 2018, andJanuary 4, 2019). The 2D-KZ filter was applied to theimage of LOS deformations to smear the rapid
intensity changes and extract smooth contour lines(Fig. 2). Fig. 2 shows during the three months that thewidespread area west of the irrigation canal had been
deforming with ridges developing about 15 mm highand troughs sunken about 20 to 30 mm deep. The
troughs drawn with blue contour lines seem to havedeveloped on the depositional areas of the extensive
flow slides and along the meandering trace of PaluRiver.
This pattern of LOS displacements contains errors
arising from the uncertainty of estimated orbit or
topography and the delay caused by the disturbanceof water vapor in the atmosphere. However, the LOS
displacements are remarkable, mainly on the westernside of the irrigation canal. Thus, this pattern triggers
us to acknowledge the statistical significance of the2d-KZ filtered image of the LOS displacements.

ACKNOWLEDGEMENT: This work was
supported by the Fund for the Promotion of Joint
International Research (Fostering Joint InternationalResearch (B)) “Field and laboratory investigations onliquefaction-induced flow-failure mechanism ingently sloped ground,” (Principal investigator:Takashi Kiyota), Japan Society for Promotion ofScience (JSPS). The Digital Terrain Models of thePalu Basin before and after the earthquake wereprovided by the Project for Development of Regional
Disaster Risk Resilience Plan in Central Sulawesi in
the Republic of Indonesia, Japan International
Cooperation Agency (JICA)10). The Line-of-sight(LOS) deformations over the Palu Basin obtained
from ALOS 2/PALSAR 2 were analyzed at theRemote Sensing Technology Center of Japan(RESTEC).
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4. SUMMARYThe devastating Mw 7.5 Palu Earthquake andTsunami struck northwestern Sulawesi, Indonesia, on
September 28, 2018. This earthquake's mostspectacular and catastrophic aspect was lateral slidesof the almost flat alluvial soil deposit along the west
and east bounds of the basin. To get a perspective on
the extensive ground deformation of Palu Basin, weused two sets of topographic data. Namely, (1)Digital Terrain Models (DTMs) of the Palu Basinbefore and after the earthquake and (2) Line-of-sight(LOS) deformations over the Palu Basin obtained
from ALOS 2/PALSAR 2 on two different days after
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